How to Determine A Foundation Whether to be Designed as Flexible or Rigid Body?

In this post we will discuss about strip footing. These
types of foundations are suitable where columns in one grid are near to be
founded by individual foundations beneath respective column or shear wall etc.
In case of site conditions or building geometries where transfer of lateral
load to exterior columns is required, thesefoundations are used.

It is the stiffness of the supporting structures which can
restrict differential settlement of individual column.But there the footings have to be designed as
rigid body having linear soil pressure distribution. A comparison is made
between soil stiffness and followings:

-combination of stiffness of the foundation

-framing member of superstructure

-shear wall

to predict approximate stiffness of the structure.

Now we will go through right point: When a foundation can be
treated as flexible or rigid. In the above comparison a relative stiffness is
determinedwhich is termed as Kr.
The Kr value indicates the rigidity or flexibility assumption to be
made for designing. Meyerhof in 1953 gave formula about relative stiffness. These
are

This is interesting to notice that when Kr value
increases a rapid decrease in differential settlement is observed. We provide a
table below about the relation between Kr and settlement values
below:

Footing size

Relative stiffness

(Differential settlement)/(Total settlement)

Long

Kr=0

0.5

Square

Kr=0

0.35

-

Kr=0.5

0.1

When a relative stiffness, derived from above equations or
analysis, becomes o.5, the foundation are assumed rigid in design i.e.
variation in soil pressure are derived by simple statics. But if relative
stiffness is found less than 0.5 the foundation should be designed as flexible
considering a approach of foundation modulus.